selftests/bpf: Cover generic devmap egress last-dst rewrite

Strengthen xdp_veth_egress to check that each destination observes the
MAC selected for its own egress ifindex, instead of only checking that
the observed MAC differs from a single magic value.

Add a generic XDP last-destination test where an earlier destination does
not have a devmap egress program while the final destination does. This
covers the case where the final destination runs on the original skb and
could otherwise rewrite packet data still shared with an earlier cloned
skb.

Use deterministic DEVMAP_HASH keys for the egress map so the intended
last destination is stable. Initialize the result map with a sentinel
value and check that store_mac_1 overwrites it before checking that the
earlier destination did not observe the MAC written by the final
destination.

Suggested-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
Link: https://lore.kernel.org/r/20260612114032.244616-3-sun.jian.kdev@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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